Thermal Management in a Scramjet-Powered Hypersonic Cruise Vehicle

Abstract

Due to the large aerodynamic heating at high Mach numbers, Thermal Protection System (TPS) design considerations are critical for hypersonic vehicles, and engineers seek to incorporate heating constraints earlier in the design process. Preliminary design studies necessitate the use of low-fidelity tools for design and optimization purposes. A number of low-fidelity models exist in the open literature for full scramjet-powered hypersonic vehicles, and some of these models incorporate passive TPS models (where the material on the outer surface of the vehicle absorbs energy, preventing the energy from seeping into the structure). However, none of the models incorporate an active TPS (where the fuel is used as a coolant in the heat exchangers surrounding the engine) in addition to a passive TPS model. In the present work, active and passive TPS models were added to a full scramjet-powered vehicle model developed at the University of Michigan. For a trimmed hypersonic waverider vehicle, computations were performed to investigate the operability limits that occur due to excessive heating of the external surface, including the nose region and combustor wall region, and the heating of the hydrogen fuel, which is used as coolant. The operability limits computed include the maximum values of flight Mach number, dynamic pressure and the flight time before one of several temperature limits is exceeded. To compute operability limits, efficient aerodynamic heating and TPS models were added to the reduced order model MASIV which contains an advanced combustion analysis and a trim code. Results show the effects of varying the thickness of the three-layer thermal protection system that consists of a radiation shield, an insulation layer and the vehicle wall. Regarding the active cooling system, the heat exchanger heat flux is modeled assuming the hydrogen fuel is a supercritical fluid and lookup tables for the fuel properties at supercritical conditions are incorporated. Recirculating the heated fuel back into the fuel tank raises the fuel temperature and decreases the fuel density (increasing the volume); the analysis computes the maximum flight time before the fuel tank temperature and fuel volume exceed acceptable limits. By extending the active cooling system to a small region of the inlet (instead of just around the isolator and combustor), the operability limits are increased from a flight Mach number of 7.3 to 8.6. Optimizations for the active and passive thermal protection systems are performed. For the passive thermal protection system, the optimal insulation thickness distributions are found which minimize the insulation mass while still ensuring that the titanium skin remains below its failure temperature. At a 40 minute cruise at Mach 6 and 80 kPa free-stream dynamic pressure, the optimized insulation mass is 74 percent less than the initial condition. For the active TPS, the parameters impacting the final fuel temperature are optimized to find the minimum fuel temperature at the end of a 40 minute cruise. The coolant mass flow rate is one parameter considered in the active cooling system optimization. For cruise at Mach 8 and 60 kPa free-stream dynamic pressure, the change in coolant mass flow rate over time is first represented as a linear decrease and is later represented by a quadratic one. It is found the final fuel temperature in the quadratic case is 19 percent less than the linear case.